Fetching currency data is not phoning home. And how is currency conversion supposed to work without contacting an external service?
Let's not dilute the original meaning. Phoning home means sending telemetry and other usage data to its creator and associated third parties, and this data exchange is not at all necessary for the program to do its job.
Communicating with the Internet without express prior consent, with as much emphasis as I can convey in the limited formatting options provided by this site, ABSOLUTELY IS PHONING HOME. Any other assumption is unsafe and a privacy hole large enough to deliver freight.
If the calculator wants to pull exchange rate information, all it needs to do is ask. Anything else should be presumed as shady in the current environment.
Let's not dilute the original meaning. Phoning home means sending telemetry and other usage data to its creator and associated third parties, and this data exchange is not at all necessary for the program to do its job.